These other factors are background, class or age.
This phenomenon called assortative mating , the researchers state, may also be a reason why obesity is on the rise, for kids of overweight couples are likely to inherit genes that make them prone to piling on the pounds too.
"If someone who is overweight or obese gets together or marries someone who is overweight or obese, their children could have a genetic disposition to obesity," the Daily Mail quoted one of the researchers, Dr Diane Jackson, as saying.
The study was conducted on a group of 42 couples, who the researchers found, shared similar levels of body fat. They noted that instead of simply putting on weight together after marriage, the couples were already fat at the time they met.
The boffins also said that this phenomenon was also something that had grown over the past 40 to 50 years.
"In the 1940s and 1950s, people mostly got married in their early twenties before they were overweight or obese," said researcher Professor John Speakman.
"So it would have been difficult for them to assortatively mate for body fatness because it would have been impossible to distinguish somebody who was thin from somebody who was thin but going to become fat.
"Nowadays, we choose partners and have children much later, but if we are going to become obese, on average we do so much younger.
"This makes it possible for potential partners to select each other on the basis of body fatness.
The study is published in the American Journal of Clinical Nutrition.